Electrolytic preparation of sodium



Jan. 8, 1963 J. L. KROON ETAL 3,072,544

ELECTROLYTIC PREPARATION OF SODIUM Filed Nov. 9. 1959 COMPOS/T/O SIN MOLE PERCENT 1N VEN TORS.

James L. Kroon Rober/ 0. Blue BY W United States Patent Ofifice 3,072,544 Patented Jan. 8, 1963 3,072,544 ELECTROLYTIC PREPARATION OF SODIUM James L. Kroon and Robert D. Blue, Midland, Mich., as-

signors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware 1 Filed Nov. 9, 1959, Ser. No. 851,838 3 Claims. (Cl. 204-68) The invention relates to the preparation of sodium and more particularly relates to an improved electrolytic process for the preparation of sodium.

For about the last 35 years the electrolytic production of sodium in metallic form has been carried out by electrolyzing a fused mixture consisting of sodium chloride and calcium chloride in about equimolar proportions. The electrolysis is in most cases conducted in a cell of the type devised by Downs and described in US. Patent 1,501,756. Although the process of electrolyzing a fused mixture of sodium chloride and calcium chloride has been widely used commercially for a long period and in general is considered satisfactory it does have some disadvantages. For example, the requisite operating temperature range of about 575 to 595 C. introduces limitations in container materials employed in cell construction and, further, calcium is for-med under the present and practical modes of operation as a solid. This presents particular difiiculties in handling the metal formed by electrolysis. Specifically, solid calcium and calcium in solution in the sodium enters the riser pipe and is trapped by cooling with various devices and returned to the cell by mechanical disengagement from the cooling surfaces. Such disengagement necessarily is performed at frequent intervals during cell operation. This adds additional expense to the cost of production. Likewise, since a considerable amount of the calcium accumulated in the cell is in the solid form, it plates out on the cathode eventually forming a calcium sludge which gradually bridges over to the diaphragm. This causes short-circuiting or otherwise reduces the current efficiency of the cell. Further, the removal of calcium in the purification of product sodium is a problem. It is therefore highly desirable to the industry to overcome these and other disadvantages of the present commercial process and to provide an improved method of producing sodium. It is therefore an object of the invention to provide an improved process for the production of sodium.

Another object is to provide an improved electrolytic process for the production of sodium.

Another object of the invention is to provide an improved electrolytic process for the production of substantially calcium-free sodium.

Still a further object of the invention is to provide an improved electrolytic process for the production of sodium which may be carried out at a temperature in the range of about 400 to 500 C.

These and other objects and advantages of the inven tion will be more fully understood on becoming familiar with the following specification and drawing and the appended claims. a

The invention is predicated on the discovery that upon electrolyzing a low melting saline composition consisting substantially exclusively of .the chlorides of sodium, potassium, and lithium, the proportion of sodium chloride comprising less than about 25 mole percent of the saline composition, sodium is produced in surprising purity, in excess of about 80 weight percent, and in fact is readily obtainable in 95 weight percent purity or better, even though sodium chloride is a minor component of the cell electrolyte.

With reference to the drawing the single FIGURE is a trilinear graph having illustrated thereon a trapezoidal area defined by the lines connecting points A, B, C and D and designated by the letter E. Area E represents a range of suitable ternary saline mixtures employed as the cell electrolyte according to the present electrolytic process. A smaller area F, within area E and bounded by the curve G and the portion HI of the line AD, represents a preferred range of ternary saline mixtures employed in the said process.

As aforesaid, the low melting saline composition electrolyzed according to the process of the invention consists essentially of a mixture of the chlorides of sodium, lithium and potassium. Suitable ranges of proportions of each chloride falling within area B of the drawing are listed respectively as follows: From 5 to 25 mole percent of sodium chloride, 20 to 55 mole percent of potassium chloride and from 40 to mole percent of lithium chloride. The preferred range of ternary compositions is defined by the area F of the drawing. These ternary mixtures which exhibit melting points in the range of about 360 to 500 C. are lower melting than sodium chloride-calcium chloride compositions, thus permitting greater latitude in the selection of materials of construction for the electrolytic cell employed in the present process. A particularly preferred electrolytic composition consists of about 50 mole percent of lithium chloride, 35 mole percent of po tassium chloride, the balance sodium chloride.

Many types of electrolytic cells and procedures can be employed in conducting the process of the present invention. In general the principal requirements are a suitable vessel for containing the electrolyte, having an anode and a cathode positioned therein and separated by a diaphragm and being enclosedbut having a means for supplying electrolyte thereto, recovery of the gas formed, recovery of the metal formed, and, where necessary, a receiving chamber for collecting the cathode product prior to separation of the sodium from the lithium and potassium by a suitable procedure. A particularly efiicient and preferred cell is that which is basically the Downs cell described -in US. Patent 1,501,756.

In conducting the electrolysis of the invention, the current density at the anode can be varied over a wide range. In general, current densities between 250 and 550 amps;/ sq. ft. are satisfactory though higher current densities may be employed, if desired. Generally the cathode current density is maintained approximately the same as .the anode.

The purity of the salts employed in the melt is not critical. For example, they may be contaminated by other salts, such as the salts of other metals as calcium,

magnesium and the like halides; or oxides of these and other metals including potassium, lithium and sodium 'al though in minor amounts, generally less than about 0.1 percent by weight. It is particularly preferred that the oxides contained in the melt be below about 0.02 weight percent in order to eliminate reduction in current efiiciency. It is preferred that the total amount of these and other contaminants in the salt is below about 3 percent. During operation the electrolytic cell is enclosed. Such operation is preferred to exclude moisture. Alternatively or in conjunction therewith a blanket of dry gas such as argon, helium, nitrogen and the like can be employed to prevent contamination. The electrodes can be the same or different and constructed of various materials such as carbon, iron, steel, nickel, etc. It is preferred to employ a carbon anode and an iron cathode.

The cathode product collected .in the product receiver is generally transferred to appropriate and well known distillation equipment and there fractionally distilled to recover a purified sodium metal product. Desirably two stills are employed. Two fractions are separated by the first still, namely a potassium-rich sodiumpotassium mixture and a potassium-free sodium-lithium mixture. The potassium-free sodium-lithium mixture serves as boiler feed for the second still from which are obtained two fractions, purified sodium and a lithium-rich sodium-lithium mixture. The potassium and lithium containing fractions are advantageously returned to an operating cell to equilibrate with the cell electrolyte and become cell metal again. By proper application of fractional distillation techniques to either the cathode product or the said potassium and lithium-containing fractions, purified potassium metal and purified lithium metal may also be separated and recovered as valuable by-products. In any event, to maintain good current efiiciency it is important that the loss of the electrochemical values of potassium and of lithium be avoided.

During the operation of the electrolytic cell part of the sodium chloride content of the cell electrolyte is, of course, consumed in the formation of product sodium. Much smaller amounts of lithium chloride and potassium chloride are likewise electrolyzed as well as lost by sublimation, sludge formation, entrainment in the metal product and entrainment in the spray of the liberated chlorine. Some lithium and potassium may be returned to the cell in the form of potassium or lithium-rich sodium according to one scheme of operation. But in any event net losses of each of the three chlorides are made up from time to time by sampling the cell electrolyte, analyzing it to determine the composition and adding to the cell the requisite amount of each salt, in anhydrous form, to keep the composition of the cell electrolyte within a predetermined range according to the invention.

To illustrate the process of the present invention, a number of tests were carried out in which various saline electrolyte compositions, operating temperatures, and current densities were employed. In each test the saline composition selected was placed in a steel pot with steel cathodes projecting from the sides and a graphite anode rising from the bottom. Heat was supplied to melt the mixture and the cover, chlorine collector and a metal collector was put into position on the steel pot. The melt was then purified by electrolyzing at a low voltage until all impurities readily electrolyzed were removed. The voltage was then raised and current put through at a density sufficient to cause electrolysis of the melt. At the end of about 6 hours of operation of the cell, the metal was removed from the cell by vacuum transfer. An inert gas atmosphere was maintained over the cell at all times. The electrolyte composition, the composition of the product, the operating temperature and the current density employed in each test are listed in the table. It can be seen that by increasing the relative amount of sodium chloride the percentage of lithium codeposited decreases. However as the sodium chloride content of the electrolyte is raised above about 10 mole percent the melting temperature of the electrolyte becomes increasingly higher.

Among the advantages of the process of the invention are: the flexibility in selection of container materials for cell construction, the freedom from clogging of lines used in transferring the low melting cathode product metal out of the receiver, and the relative ease of producing highly pure sodium from the product metal,

Electrolyte composi- Product composition Opera- Current. Test tion, mole percent weight percent ating density No. temperat anode,

ature, amps/it. LiCl KCl NaCl Li K Na C.

61. 5 29. 2 9. 4 11.8 0. 6 Bal. 500 432 62.0 28. (5 9. 5 10. 3 1. 4 13:11. 500 288 50. 5 34. 3 15.2 2. 1 1. 5 13:11. 500 432 52. 2 32. 4 15. 5 1. 3 1. 6 Bal. 490 288 52. 2 32.4 15. 5 1.0 1.0 Bill. 450 540 52. 6 34. 9 12. 8 4. 4 1. 1 Bal. 450 540 50. 3 35. 7 14. 0 2. 0 1. 5 Bal. 450 446 What is claimed is:

1. The process for the production of sodium which comprises electrolyzing a fused saline mixture consisting substantially exclusively of the chlorides of sodium, potassium and lithium at a temperature at least 10 centigrade degrees above the melting temperature of the said saline mixture but below about 510 C. and at a current density between 250 to 550 amps. per square foot, the proportions of the components of said saline mixture being in the range of from 5 to 25 mole percent of sodium chloride, from 20 to mole percent of potassium chloride and from 40 to mole percent of lithium chloride and the range of total composition being defined by the area defined by the lines connecting the points A, B, C and D of the drawing.

2. The process for the production of sodium metal which comprises electrolyzing a fused saline mixture consisting substantially exclusively of the chlorides of sodium, potassium and lithium at a temperature at least 10 centigrade degrees above the melting temperature of the said saline mixture but below about 510 C. and at a current density between 250 to 550 amps. per square foot, said fused saline mixture containing from about 5 to 25 mole percent of sodium chloride, the balance potassium chloride and lithium chloride in the range of total compositions being represented by the area defined by the lines connecting the points A, B, C and D of the drawing, thereby forming a molten cathode product comprising a solution of potassium and lithium metals in sodium metal, withdrawing said cathode product, said cathode product having a sodium content of at least weight percent and subjecting said cathode product to fractional distillation whereby a purified sodium metal product is obtained.

3. The method as in claim 2 in which the said withdrawn cathode product is subjected to fractional distillation whereby a potassium rich fraction and a lithium rich fraction and a purified sodium fraction are obtained and recycling said potassium and lithium-containing fractions to an operating cell.

References Cited in the file of this patent UNITED STATES PATENTS 464,097 Grabau Dec. 1, 1891 1,375,330 Smith et al Apr. 19, 1921 2,876,181 Wood Mar. 3, 1959 2,880,156 Benner et al Mar. 31, 1959 

1. THE PROCESS FOR THE PRODUCTION OF SODIUM WHICH COMPRISES ELECTROLYZING A FUSED SALINE MIXTURE CONSISTING SUBSTANTIALLY EXCLUSIVELY OF THE CHLORIDES OF SODIUM, POTASSIUM AND LITHIUM AT A TEMPERATURE AT LEAST 10 CENTIGRADE DEGREES ABOVE THE MELTING TEMPERATURE OF THE SAID SALINE MIXTURE BUT BELOW ABOUT 510*C. AND AT A CURRENT DENSITY BETWEEN 250 TO 550 AMPS. PER SQUARE FOOT, THE PROPORTIONS OF THE COMPONENTS OF SAID SALINE MIXTURE BEING IN THE RANGE OF FROM 5 TO 25 MOLE PERCENT OF SODIUM CHLORIDE, FROM 20 TO 55 MOLE PERCENT OF POTASSIUM CHLORIDE AD FROM 40 TO 75 MOLE PERCENT OF LITHIUM CHLORIDE AND THE RANGE OF TOTAL COMPOSITION BEING DEFINED BY THE AREA DEFINED BY THE LINES CONNECTING THE POINTS A, B, C AND D OF THE DRAWING. 